Fans are utilized on gasoline and diesel engines for drawing air through a radiator to assist in cooling the engine. These fans are generally operated by mounting onto a pulley-driven water pump, by direct mounting on the engine crankshaft, or by mounting on a separate fan drive. In most applications, molded plastic fans are preferred because of their light weight requiring less power from the engine or motor.
These plastic fans are subject to large forces, especially in diesel engines used to power large trucks. Rapid acceleration and deceleration of the fan due to piston firing and periodic blasts of air impacting the fan blades as the air enters the engine compartment and drive train vibrations passing through the engine structure cause flexing of the fan blades. In addition, operation of the fan at its resonance frequency will quickly cause the fan blades or the entire assembly to crack and break. This phenomenon occurs when the frequency of an external, periodic driving force, in this case the firing of the engine pistons and the vibration of the drive train from the wheels to the engine, matches the natural free oscillation frequency of the fan, resulting in a large amplitude of oscillation of the blades. Finally, stresses are also placed on the fan from the tension of pulley belts and fluctuations in temperature.
In an attempt to strengthen the plastic fan blades, prior art plastic blades have been molded to a metal hub. Upstanding plastic ribs have also been formed on the hub and attached to the roots of the leading and trailing edges of each blade to brace the blade against flexing. To further strengthen the blades, a ring of molded plastic has been formed around the fan hub between the roots of adjacent blades on both sides of the hub. This particular configuration has several drawbacks. First, the intersection of each rib, blade, and ring forms a large mass of plastic that has molded-in stress points. During operation, this mass easily develops stress cracks, eventually resulting in total failure and breaking apart of the fan. In addition, the fan blades tend to break at the root due to severe blade vibration and flexing that occurs as a result of operating the fan at its resonance frequency. Finally, since these prior art fans are bulky and cannot be nested, they require large amounts of space for storage and shipping.